1. FIeld of Invention
This invention relates to fabrication of optical waveguide structures, and more particularly to fabrication of monolithic thin-film optical waveguides having a highly uniform thin-film thickness suitable for use with electrooptic and acousto-optic processing of high power infrared (IR) lasers.
2. Description of the Prior Art p The use of infrared (IR) lasers in both electrooptic and acousto-optic applications in applied optical systems such as optical imaging radars, high-data-rate communication systems, and high resolution spectroscopy systems is well known in the art. The desirability of using optical waveguides for generating intelligence derives both from the enormous inherent bandwidth of optical devices, and from the many specialized features of optical waveguides. These optical waveguide devices can be made smaller, cheaper, and more reliable than their conventional counterparts. They ar less susceptible to information degradation from electromagnetic interference, vibration, temperature changes, and cross talk; and for specific defense applications, they offer the important advantages of electrical isolation and increased communications security. Such applications require laser signal processing, such as amplitude, frequency and phase modulation to encode intelligence information on the carrier, or deflection and switching of the laser carrier to provide discrete optical control functions. The signal processing of the laser is provided during guided mode propagation of the laser through optical waveguides comprised of a high resistivity, high index of refraction crystal material which is either eptiaxially grown to a desired dimensional thickness, or fabricated through mechanical thinning and polishing of a larger ingot crystal to the desired dimension. Such epitaxially grown, or mechanically fabricated optical waveguides have been provided for use with infrared lasers only at very low optical power levels. For broadband signal processing the laser input power and/or the electrical driving power of the processing signal source must be increased. With increasing power levels, the prior art optical waveguide devices suffer performance degradation from a variety of waveguide imperfections. Some of the imperfections are pervasive to the waveguide material medium itself, such as free carrier absorption, dislocation, and lattice mismatch. Other imperfections are associated with the waveguide fabrication, such as processed induced damage resulting in minute fracture or surface imperfections of the crystal structure, and the lack of thickness uniformity throughout the surface area of the waveguide. Still further imperfections are associated with waveguide design, such as electric power loss at high frequencies resulting from improper electrooptic interface, surface deformation of the waveguide by electrode mounting, and stress-induced birefringence caused by a discontinuity in the index of refraction of the waveguide medium along the boundaries of the electrode resulting in optical distortion.
Epitaxially grown waveguide structures are limited in useful applications since only one major surface of the grown waveguide structure is available for electrode deposition, as shown in the use of epitaxially grown optical waveguides in laser switching and deflection applications as disclosed in U.S. Pat. No. 3,904,270, entitled INTEGRATED OPTICAL SIGNAL PROCESSING SYSTEM, issued to P. K. Cheo on Sept. 9, 1975. In addition, the generic substrate material on which the waveguide medium is grown has the same basic crystalline structure and a closely related index of refraction as that of the grown layer, resulting in undesirable leakage of the guided optical wave into the substrate. The mechanically thinned waveguide structures known in the prior art provide very low optical transmission and are limited to very small size and propagation path length due to the prior art methods of fabricating the thin-film waveguides from bulk crystal ingots. The known prior art devices are limited to only the planar configuration due to inadequate thickness control, and thickness uniformity throughout the surface area of the waveguides.